In North America, telephone switching systems have been developed which use ferreed crosspoints in the switching network. In at least one such system, designated No. 1 ESS (electronic switching system), battery and ground are removed from the switching network and consequently from any associated telephone line during the switching interval in order to protect the ferreed switches. Certain station equipments at the terminal end of the telephone line contain circuits which are held operated by current supplied from the central office. As an example, a key telephone system requires a substantially continuous source of battery voltage on the telephone line in order to maintain the system in a hold condition. The break on the telephone line which results during the above-described switching interval is sufficient to disconnect the hold circuit of the key telephone system which results in some calls being lost when they are involved in network switching operations. The interval during which battery and ground are removed from the telephone line is known as the open switching interval and any system which is designed to prevent calls being lost due to this problem is referred to as open switching interval protection (OSIP). Hence, the function of an OSIP circuit is to provide battery voltage on the telephone line during the open switching interval.
Various such circuits have been proposed in the past. One arrangement uses a circuit bridged across the telephone line which is controlled by software in the electronic switching system. In this arrangement, an auxiliary source of battery and ground are bridged across the telephone line through current limiting resistors via relay contacts which are under direct control of the switching system program. For this reason the circuit control adds a "real time" load on the system program thus reducing the call carrying capacity of the machine.
In an alternate proposal, a detector circuit bridged across the output of the electronic switching system is arranged to control a relay which again connects the auxiliary source of battery voltage in shunt with the line through current limiting resistors. However, in order that the auxiliary source will not lock up its own detector circuit, so that the latter cannot sense when voltage from the switching system is restored to the telephone line, additional contacts on the relay break the telephone line between the detector circuit and the auxiliary source bridged across the telephone line. This alternate arrangement has several disadvantages. Firstly, it is necessary to cut into the telephone line to connect the break contacts of the relay in series therewith. Secondly, because the telephone line is broken by these series connected break contacts when the auxiliary source is applied to the telephone line, the station equipment is disconnected from the central office switching system during a crucial "power-cross test" which occurs on every call origination. This test is performed in order to detect the presence of a high voltage (such as 117 volts a-c from the local power company) which may become crossed to the telephone line in the environment outside the central office switching system.